including unit definitions, keeps on changing with time. Whenever the accuracy of measurement of a particular unit was enhanced substantially by adopting new principles, member nations of metre treaty (signed in ), agreed to change the formal definition of that unit. Each modern industrialised country, including India, has a National Metrology Institute (NMI), which maintains standards of measurements. This responsibility has been given to the National Physical Laboratory (NPL), New Delhi.
This laboratory establishes experiments to realise the base units and derived units of measurement and maintains National Standards of Measurement. These standards are periodically inter-compared with standards maintained at other National Metrology Institutes in the world, as well as those, established at the International Bureau of Standards in Paris. Table . Base Physical Quantities and their Units Base Physical Quantity Symbol for Quantity Name of SI Unit Symbol for SI Unit Length Mass Time Electric current Thermodynamic temperature Amount of substance Luminous intensity l m t I T n I v metre kilogram second ampere kelvin mole candela m kg s A K mol cd These prefixes are listed in Table .
. Let us now quickly go through some of the quantities which you will be often using in this book. The definitions of the SI base units are given in Table . .
The SI system allows the use of prefixes to indicate the multiples or submultiples of a unit. Table . Definitions of SI Base Units Unit of length metre The metre , symbol m is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299792458 when expressed in the unit ms – , where the second is defined in terms of the caesium frequency V Cs .
Unit of mass kilogram The kilogram, symbol kg. is the SI unit of mass. It is defined by taking the fixed numerical value of the planck constant h to be .62607015× – when expressed in the unit Js, which is equal to kgm s – , where the metre and the second